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27 records · Page 2
All sourcesNASA NTRSDOE OSTI
NASA NTRS1993

High-frequency techniques for RCS prediction of plate geometries and a physical optics/equivalent currents model for the RCS of trihedral corner reflectors

Part 1 of this report continues the investigation, initiated in previous reports, of scattering from rectangular plates coated with lossy dielectrics. The hard polarization coefficients given in the last report are incorporated into a model, which includes second- and third-order diffractions, for the coated plate. Computed results from this model are examined and compared to measured data. A breakdown of the contribution of each of the higher-order terms to the total radar cross section (RCS) is given. The effectiveness of the uniform theory of diffraction (UTD) model in accounting for the coating effect is investigated by examining a Physical Optics (PO) model which incorporates the equivalent surface impedance approximation used in the UTD model. The PO, UTD, and experimental results are compared. Part 2 of this report presents a RCS model, based on PO and the Method of Equivalent Currents (MEC), for a trihedral corner reflector. PO is used to account for the reflected fields, while MEC is used for the diffracted fields. Single, double, and triple reflections and first-order diffractions are included in the model. A detailed derivation of the E(sub theta)-polarization, monostatic RCS is included. Computed results are compared with finite-difference time-domain (FDTD) results for validation. The PO/MEC model of this report compares very well with the FDTD model, and it is a much faster model in terms of computational speed.

Balanis, Constantine A.↗
NASA NTRS1988

Theory and application of radiation boundary operators

A succinct unified review is provided of the theory of radiation boundary operators. With the recent introduction of the on-surface radiation condition (OSRC) method and the continued growth of finite-difference and finite-element techniques for modeling electromagnetic wave scattering problems, the understanding and use of radiation boundary operators has become increasingly important. Results are presented to illustrate the application of radiation boundary operators in both these areas. Recent OSRC results include analysis of the scattering behavior of both electrically small and large cylinders, a reactively loaded acoustic sphere, and a simple reentrant duct. Radiation boundary operator results include the demonstration of the effectiveness of higher-order operators in truncating finite-difference time-domain grids.

Moore, Thomas G.↗
NASA NTRS2003

Diffraction Efficiency of Thin Film Holographic Beam Steering Devices

Dynamic holography has been demonstrated as a method for correcting aberrations in space deployable optics, and can also be used to achieve high-resolution beam steering in the same environment. In this paper, we consider some of the factors affecting the efficiency of these devices. Specifically, the effect on the efficiency of a highly collimated beam from the number of discrete phase steps per period is considered for a blazed thin film beam steering grating. The effect of the number of discrete phase steps per period on steering resolution is also considered. We also present some result of Finite-Difference Time-Domain (FDTD) calculations of light propagating through liquid crystal "blazed" gratings. Liquid crystal gratings are shown to spatially modulate both the phase and amplitude of the propagating light.

Titus, Charles M.↗
NASA NTRS1991

Advanced electromagnetic methods for aerospace vehicles

During this period the research program addressed the following three topics: (1) composite materials; (2) precipitation static (P-Static); and (3) antenna technology. On the topic of Composite Materials our main efforts were directed toward making measurements on several new samples of composite materials made available to ASU by Stanford Research Institute (SRI) through the efforts of Mr. Frank Casler of AVRADA. These samples can be classified into three distinct materials with each material having its own distinct electrical properties. In addition, attempts were made to make predictions of the effects on antenna patterns by composite materials. This will take a greater emphasis in the next reporting period. In Precipitation Static (P-Static), the main effort was devoted toward developing a Voltage Finite-Difference Time-Domain computer code to account for the voltage variation on a conducting body as the primary source of corona discharge, instead of the electric field. Due to complexities stemming from the interactions between the potentials, the fields, and current sources, the decision was made to begin with a simple two-dimensional problem without the corona discharge and check our programs in a series of simple models, culminating in the full corona discharge problem. This report deals with the first stage of such development. During this reporting period, the main effort in Antenna Technology was toward the design, fabrication, and testing of a cavity-backed slot antenna using ferrite material. Using the ferrite material available to us during this period, the resonances of this antenna were around 5 and 8 GHz. Attempts will be made to model such an antenna and to lower its resonance down into the VHF and UHF bands.

Balanis, Constantine A.↗
NASA NTRS1994

High-frequency techniques for RCS prediction of plate geometries and a physical optics/equivalent currents model for the RCS of trihedral corner reflectors, parts 1 and 2

Formulations for scattering from the coated plate and the coated dihedral corner reflector are included. A coated plate model based upon the Uniform Theory of Diffraction (UTD) for impedance wedges was presented in the last report. In order to resolve inaccuracies and discontinuities in the predicted patterns using the UTD-based model, an improved model that uses more accurate diffraction coefficients is presented. A Physical Optics (PO) model for the coated dihedral corner reflector is presented as an intermediary step in developing a high-frequency model for this structure. The PO model is based upon the reflection coefficients for a metal-backed lossy material. Preliminary PO results for the dihedral corner reflector suggest that, in addition to being much faster computationally, this model may be more accurate than existing moment method (MM) models. An improved Physical Optics (PO)/Equivalent Currents model for modeling the Radar Cross Section (RCS) of both square and triangular, perfectly conducting, trihedral corner reflectors is presented. The new model uses the PO approximation at each reflection for the first- and second-order reflection terms. For the third-order reflection terms, a Geometrical Optics (GO) approximation is used for the first reflection; and PO approximations are used for the remaining reflections. The previously reported model used GO for all reflections except the terminating reflection. Using PO for most of the reflections results in a computationally slower model because many integrations must be performed numerically, but the advantage is that the predicted RCS using the new model is much more accurate. Comparisons between the two PO models, Finite-Difference Time-Domain (FDTD) and experimental data are presented for validation of the new model.

Balanis, Constantine A.↗
NASA NTRS1984

Application of the Green's function method for 2- and 3-dimensional steady transonic flows

A Time-Domain Green's function method for the nonlinear time-dependent three-dimensional aerodynamic potential equation is presented. The Green's theorem is being used to transform the partial differential equation into an integro-differential-delay equation. Finite-element and finite-difference methods are employed for the spatial and time discretizations to approximate the integral equation by a system of differential-delay equations. Solution may be obtained by solving for this nonlinear simultaneous system of equations in time. This paper discusses the application of the method to the Transonic Small Disturbance Equation and numerical results for lifting and nonlifting airfoils and wings in steady flows are presented.

Tseng, K.↗
NASA NTRS1985

A first-order time-domain Green's function approach to supersonic unsteady flow

A time-domain Green's Function Method for unsteady supersonic potential flow around complex aircraft configurations is presented. The focus is on the supersonic range wherein the linear potential flow assumption is valid. The Green's function method is employed in order to convert the potential-flow differential equation into an integral one. This integral equation is then discretized, in space through standard finite-element technique, and in time through finite-difference, to yield a linear algebraic system of equations relating the unknown potential to its prescribed co-normalwash (boundary condition) on the surface of the aircraft. The arbitrary complex aircraft configuration is discretized into hyperboloidal (twisted quadrilateral) panels. The potential and co-normalwash are assumed to vary linearly within each panel. Consistent with the spatial linear (first-order) finite-element approximations, the potential and co-normalwash are assumed to vary linearly in time. The long range goal of our research is to develop a comprehensive theory for unsteady supersonic potential aerodynamics which is capable of yielding accurate results even in the low supersonic (i.e., high transonic) range.

Freedman, M. I.↗
NASA NTRS1990

Hypercube-Computer Analysis Of Electromagnetic Scattering

Capabilities of hypercube and parallel processing demonstrated. Report describes use of Mark III Hypercube computer to analyze scattering of electromagnetic waves. Purpose of study to assess utility of parallel computing in such computation-intensive problems as large-scale electromagnetic scattering. Two electromagnetic codes based on different algorithms converted to run on Mark III Hypercube. First code implements finite-difference, time-domain solution of Maxwell's curl equations. Second code is Numerical Electromagnetics Code (NEC-2) which embodies frequency-domain method and developed to analyze electromagnetic responses of antennas and other metallic structures. On Mark III Hypercube with 32 active nodes, largest lattice contains about 2,048,000 unit cells.

Patterson, J. E.↗
NASA NTRS2001

Retrieval of Polar Stratospheric Cloud Microphysical Properties From Lidar Measurements: Dependence on Particle Shape Assumptions

Knowledge of particle sizes and number densities of polar stratospheric clouds (PSCs) is highly important, because they are critical parameters for the modeling of the ozone chemistry of the stratosphere. In situ measurements of PSC particles are rare. the main instrument for the accumulation of PSC data are lidar systems. Therefore the derivation of some microphysical properties of PSCS from the optical parameters measured by lidars would be highly beneficial for ozone research. Inversion of lidar data obtained in the presence of PSCs formed from crystalline particles type 11 and the various nitric acid tri Ydrrate (NAT) types cannot be easily accomplished, because a suitable scattering theory for small faceted crystals has not been readily available tip to now. As a consequence, the T-matrix method is commonly used for the interpretation of these PSC lidar data. Here the assumption is made that the optical properties of an ensemble of spheroids resemble those of crystalline PSCs, and microphysical properties of the PSC are inferred from the optical signatures of the PSC at two or more wavelengths. The problem with the T-matrix approach is that the assumption of spheroidal instead of faceted particles can lead to dramatically wrong results: Usually cloud particle properties are deduced from analysis of lidar profiles of backscatter ratio and depolarization ratio. The particle contribution to the backscatter ratio is given by the product of the particle number density and the backscattering cross section. The latter is proportional to the value of the particle's scattering phase function at 180 degrees scattering angle. At 180 degrees however, the phase functions of rough, faceted crystals and of spheroids with same maximum dimension differ by a factor of 6. From this it follows that for a PSC consisting of faceted crystals, the particle number density is underestimated by roughly the same factor if spheroidal particles are unrealistically assumed. We are currently developing a retrieval technique for determining the microphysical parameters of crystalline PSCs that takes into account the faceted shape of the PSC particles. This approach utilizes finite-difference time-domain (FDTD) calculations of particle optical properties. The accuracy and the free choice of the shape of the scattering particle make the FDTD technique a promising tool for the inversion of PSC lidar data. A first comparison of FDTD and T-matrix calculations will be presented.

Reichardt, Susanne↗